1. Field of the Invention
The present invention relates to a damper device for rotary motion which is called a torsional damper and is, in one embodiment, assembled in series midway a power transmitting mechanism for automobiles. The damper device for rotary motion of the present invention prevents fluctuations in the rotational speed and torque occurred on the engine side from being transmitted to the transmission side, and also effectively damps vibrations caused by the swingback after those fluctuations.
2. Description of the Prior Art
Automatic transmissions for automobiles which include automatic clutches assembled therein instead of torque converters have been researched and practiced in several models. When an automobile equipped with such an automatic clutch starts up or shifts a gear, the speed and torque of rotary motion transmitted from an engine to a propeller shaft through a transmission are greatly fluctuated the moment the automatic clutch is connected (turned on) and disconnected (turned off). If those fluctuations in the speed and torque are directly transmitted to drive wheels, there would occur shocks, causing persons in the automobile to feel uncomfortable. To eliminate such an unpleasant feeling, it is required to assemble a damper device for rotary motion midway a power transmitting mechanism for absorbing torque fluctuations produced momentarily upon tuning-on and -off of the automatic clutch. Known damper devices for rotary motion useful to that purpose are described, e.g., in the following references (1) to (4).
(1) Japanese Patent Laid-Open No. 2-57743:
A damper device for rotary motion described in the reference (1) comprises an inner ring and an outer ring which are arranged concentrically with each other, and compression coil springs disposed between inner projections jutting on an outer peripheral surface of the inner ring and outer projections jutting on an inner peripheral surface of the outer ring. In this damper device for rotary motion, the inner and outer rings serve as, for example, input and output members, respectively. When the rotational speed and torque of the power to be transmitted are fluctuated, the compression coil springs are forced to expand and contract so that both the rings are relatively displaced in the rotating direction to absorb the fluctuations in the rotational speed and torque.
(2) Japanese Patent Laid-Open No. 63-180725:
In a damper device for rotary motion described in the reference (2), rotating forces are transmitted from an input portion to an output portion through a cam unit comprised of projections and recesses arranged in the circumferential direction with balls fitted in the recesses. When the rotational speed and torque of the power to be transmitted are fluctuated, input- and output-side cam portions are relatively displaced in the circumferential and axial directions, whereupon a pair of diaphragm springs holding the cam unit between them are elastically deformed to absorb the fluctuations in the rotational speed and torque. The fluctuations in the rotational speed and torque are also damped through the rolling resistance of the balls and the friction resistance of friction members provided adjacent the cam unit.
(3) Japanese Patent Laid-Open No. 4-236847:
A damper device for rotary motion described in the reference (3) includes a cam displaceable axially upon fluctuations in the rotational speed and torque of the power to be transmitted. The displacement of the cam elastically deforms diaphragm springs which partition a damping chamber, causing a viscous liquid to come into and out of the damping chamber. The fluctuations in the rotational speed and torque are damped with the elastic deformation of the diaphragm springs and the resistance occurred upon the viscous liquid coming into and out of the damping chamber.
(4) Japanese Patent Laid-Open No. 2-138241:
In a damper device for rotary motion described in the reference (4), an outer peripheral surface of a first rotary shaft and an inner peripheral surface of a splined tube are engaged with each other through helical splines. The splined tube is loosely fitted in an outer tube to be axially displaceable, the outer tube being fixed to the end of a second rotary shaft and filled with oil therein. With the rotation of the first rotary shaft, the splined tube is displaced axially in the outer tube while fluctuations in the rotational speed and torque of the power applied to the first rotary shaft are absorbed through the viscous resistance of the oil.
Any of the conventional damper devices for rotary motion described in the references (1) to (4) can absorb the fluctuations in the rotational speed and torque of the power applied to the rotary shaft on the input side, and can prevent the fluctuations from being directly transmitted to the rotary shaft on the output side. However, the following problems to be solved still remain with regard to, e.g., difficulties in achieving the small size and high damping performance.
First, in the damper device for rotary motion described in the reference (1), because of the structure bearing total torque of an engine by the plurality of compression coil springs, it is required to increase the wire diameter of each compression coil spring so that the torque can be borne by the compression coil springs. This results in difficulties in reducing the size and weight of the damper device for rotary motion. Particularly, in the case where the entire length of the compression coil spring is increased to enlarge the working angular range (i.e., the angular range where rotation variations can be absorbed by the damper device for rotary motion), the size of the damper device for rotary motion is increased remarkably. The reason is that the spring constant of the compression coil spring, which is reduced as a result from increasing the entire length of the compression coil spring to enlarge the working angular range, must be compensated for (prevented from lowering) by increasing the wire diameter. Here, in order to enable the compression coil springs to bear the large torque transmitted from the engine, the spring constant is required to be more than a certain value.
The damper device for rotary motion described in the reference (1) includes a damper in which a viscous liquid is interposed between planes extending perpendicularly to the axis and given with forces in the shearing direction, for the purpose of preventing the swingback of rotations after the fluctuations in the rotational speed and torque (i.e., damping reciprocal vibrations in the rotating direction). With such a structure, however, the surfaces facing each other with the viscous liquid between them must be finished into highly accurate planes free from any warps, etc., resulting in the increased machining cost. Further, the reference (1) describes that grease is employed as the viscous liquid to achieve higher damping performance. In the damper device for rotary motion which is assembled in an automatic transmission, however, it is difficult to seal off the grease from an automatic fluid (ATF). This raises the problem that a sealing device requires a large cost, or the grease may mix into the ATF and expedite deterioration of the ATF.
The above disadvantages are not limited to the damper device for rotary motion described in the reference (1), but common to all similar damper devices, i.e., damper devices for rotary motion including compression coil springs arranged in the radial direction (in point of increasing the size), and damper devices for rotary motion utilizing viscosity of grease to attain a vibration damping function (in points of difficulties in sealing-off and deterioration of the ATF).
In the damper device for rotary motion described in the reference (2), the use of the diaphragm springs increases a diameter of the entire device. Also, since the fluctuations in the rotational speed and torque are damped through the rolling resistance and the friction resistance, sufficient damping performance cannot be always achieved. Additionally, wears of the sliding friction surfaces and a reduction in the damping performance due to deterioration over time, etc. are not negligible. This makes it difficult to ensure a satisfactory durability.
In the damper device for rotary motion described in the reference (3), the use of the diaphragm springs increases a diameter of the entire device as with the damper device described in the reference (2). Also, since the hydraulic pressure near an outer periphery of the damping chamber is raised due to centrifugal forces, the damping performance is inevitably greatly affected by such a partial rise of the hydraulic pressure and achieving stable damping performance is difficult. Further, because of using a sliding cam, wears of the friction surfaces are not negligible and a satisfactory durability is not positively ensured. In addition, it is thought that the damping chamber must be made sufficiently oil tight to achieve the desired damping performance, but it is difficult to keep a cam portion, for example, to be sufficiently oil tight and to practically achieve the desired damping performance.
In the damper device for rotary motion described in the reference (4), since the portions engaging through the helical splines are frictionally engaged with each other, wear accumulated during long term use is not negligible. Also, because of the oil being sealed off in the outer tube, if the splined tube is abruptly and largely displaced in the axial direction due to abrupt and large torque fluctuations, cavitations may occur in the oil within the outer tube (on the lower pressure side), or the pressure may be excessively raised (oil pressure boost due to the pumping action). Air bubbles generated with the cavitations bring a reduction in damping performance, and the oil pressure boost due to the pumping action brings a reduction in durability of the outer tube.
Further, in any of the damper devices for rotary motion described in the references (1) to (4), the damping performance cannot be adjusted from the mechanical point of view and, therefore, a sufficient vibration damping effect cannot be always developed for the reasons (a) to (c) as follows:
(a) In the structure utilizing viscosity of a viscous liquid such as oil to dampen vibrations, the vibration damping effect is greatly varied with changes in viscosity depending on temperature changes, and stable performance cannot be achieved.
(b) Intelligent functions have recently been more and more incorporated in automobiles and, in the near future, operating conditions of an automatic transmission including an automatic clutch will be controlled depending on the engine rotational speed and the vehicle speed in many models. In this case, the damping performance of a damper device for rotary motion will be required to be finely adjusted depending on the engine rotational speed and the vehicle speed. However, the known damper devices for rotary motion described above cannot meet such a demand.
(c) There are difficulties in satisfying both damping characteristics which are required in the occurrence of low-frequency vibrations, and damping characteristics which are required in the occurrence of high-frequency vibrations. More specifically, low-frequency vibrations, which are caused by resonation between the vehicle weight and springs in a damper due to shocks generated upon turning-on and -off of the automatic clutch or shocks produced upon abrupt speed-up and slowdown, must be damped quickly. To this end, the damping effect with a viscous liquid such as oil must be increased. On the contrary, if the damping effect is increased too much, the torque transmission rate would be elevated in the range where high-frequency vibrations are applied. This would result in that high-frequency vibrations transmitted from the engine side to the damper device for rotary motion, such as torque fluctuations ascribed to lags in the engine ignition timing, are substantially directly transmitted to the downstream in the transmitting direction. Thus, the damping characteristics required for damper devices utilizing a viscous liquid, etc. may be reversed depending on the purposes. However, the conventional damper devices of the fixed structure unable to adjust the damping performance cannot sufficiently meet such requirements.